In the typical solenoid operated pilot valve, a solenoid coil serves to shift the solenoid plunger, which acts as a valving element, in one direction to block the flow of the pilot air through a port at one end of the plunger travel and a coil return spring shifts the plunger in the opposite direction upon de-energization of the solenoid coil to block the flow of pilot air through another port at the opposite end of plunger travel. The coil spring has been known to break rendering the pilot valve inoperative. In addition, after repeated usage the return spring may fatigue reducing the spring force and thereby increasing the lag time in operation of the valve. Also, the force resulting from the air pressure against the solenoid plunger is relied upon to help overcome the force of the return spring when the solenoid is energized and consequently such valves will only function with at least a given minimum air pressure.
I have discovered that the return spring for shifting the plunger in one direction upon de-energization of the solenoid coil may be eliminated and the above-mentioned disadvantages attendant from its use thereby obviated.